Stents are generally known. Indeed, the term “stent” has been used interchangeably with terms such as “intraluminal vascular graft” and “expansible prosthesis”. As used throughout this specification the term “stent” is intended to have a broad meaning and encompasses any expandable prosthetic device for implantation in a body passageway (e.g., a lumen or artery).
In the past eight to ten years, the use of stents has attracted an increasing amount of attention due to the potential of these devices to be used, in certain cases, as an alternative to surgery. Generally, a stent is used to obtain and maintain the patency of the body passageway while maintaining the integrity of the passageway. As used in this specification, the term “body passageway” is intended to have a broad meaning and encompasses any duct (e.g., natural or introgenic) within the human body and can include a member selected from the group comprising: blood vessels, respiratory ducts, gastrointestinal ducts and the like.
Initial stents were balloon deployable of self-expanding, spring-like devices which were inserted in the body passageway in a contracted state.
In the case of self-expanding, spring-like devices, when released, the stent would automatically expand and increase to a final diameter dependent on the size of the stent and the elasticity of the body passageway. An example of such a stent is known in the art as the Wallstent™.
The self-expanding stents were found by some investigators to be deficient since, when deployed, they could place undue, permanent stress on the walls of the body passageway. Further, upon expansion, the stent would shorten in length in an unpredictable fashion thereby reducing the reliability of the stent.
This led to the development of various stents which were controllably expandable at the target body passageway so that only sufficient force to maintain the patency of the body passageway was applied in expanding the stent. Generally, in the balloon-deployable systems, a stent, in association with a balloon, is delivered to the target area of the body passageway by a catheter system. Once the stent has been properly located (for example, for intravascular implantation the target area of the vessel can be filled with a contract medium to facilitate visualization during fluoroscopy), the balloon is expanded thereby expanding the stent by plastic deformation so that the latter is urged in place against the body passageway. As indicated above, the amount of force applied is at least that necessary to maintain the patency of the body passageway. At this point, the balloon is deflated and withdrawn within the catheter, and subsequently removed.
Ideally, the stent will remain in place and maintain the target area of the body passageway substantially free of blockage (or narrowing).
A stent which has gained some notoriety in the art is known as the Palmaz-SchatzT Balloon Expandable Stent (hereinafter referred to as “the Palmaz-Schatz stent”). This stent is discussed in a number of patents including U.S. Pat. Nos. 4,733,665, 4,739,762, 5,102,417 and 5,316,023, the contents of each of which are hereby incorporated by reference.
Another stent which has gained some notoriety in the art is known as the Gianturco-RoubinFlex-Stent™ (hereinafter referred to as “the Gianturco-Roubin stent”). This stent is discussed in a number of patents, including U.S. Pat. Nos. 4,800,882, 4,907,336 and 5,041,126, the contents of each of which are hereby incorporated by reference.
Other types of stents are disclosed in the following patents:    U.S. Pat. No. 5,035,706 (Gianturco et al.),    U.S. Pat. No. 5,037,392 (Hillstead),    U.S. Pat. No. 5,147,385 (Beck et al),    U.S. Pat. No. 5,282,824 (Gianturco),    Canadian Patent No. 1,239,755 (Wallsten), and    Canadian Patent No. 1,245,527 (Gianturco et al.) the contents of which are hereby incorporated by reference.
While these prior art stents have achieved a varying degree of success, the art is constantly in need of new stents having improved flexibility and stability while being able to be readily implanted with little or no trauma to the target lumen.
An improved expandable stent is described in the following copending patent applications:                Canadian Patent Application Number 2,134,997 (filed Nov. 3, 1994); Canadian Patent Application Number 2,171,047 (filed Mar. 5, 1996); Canadian Patent Application Number 2,175,722 (filed May 3, 1996); Canadian Patent Application Number 2,185,740 (filed Sep. 17, 1996); International Patent Application PCT/CA97/00151 (filed Mar. 5, 1997); and International Patent Application PCT/CA97/00152 (filed Mar. 5, 1997; the contents of each of which are hereby incorporated by reference (hereinafter collectively referred to as “the Divysio applications”). Generally, the stent illustrated in the Divysio applications comprises a tubular wall disposed between the proximal end and the distal end. The tubular wall has a longitudinal axis and a porous surface defined by a plurality intersecting members arranged to define a first repeating pattern. The first repeating pattern comprises a polygon having a pair of side walls substantially parallel to the longitudinal axis. A first concave shaped wall and a second convex-shaped wall connect the side walls. The stent is expandable from a first, contracted position to a second, expanded position upon the application of a radially outward force exerted on the stent.        
While the stents disclosed in the Divysio applications represent an advance in the art, there is still room for improvement.
One specific area in which improvement is desirable is in the treatment of ostial stenosis (these typically occur in the coronary arteries, vein grafts and renal arteries). As is known in the art (and discussed in more detail with reference in FIG. 11 in International Patent Applications PCT/CA97/00151 and PCT/CA97/00152 referred to hereinabove), ostial stenosis occurs as a result of narrowing of the ostial segment of the right coronary artery. Ideally, a stent capable of implantation into such an ostial stenosis must be of sufficient rigidity after expansion to resist the elastic recoil of the ostial blockage. However, a stent of such sufficient rigidity will likely be deficient since it will either: (i) be retarded in its advance along the artery due to the sharp bend in the right coronary artery, or (ii) traverse the sharp bend in the right coronary artery but subsequently straighten the distal portion of the right coronary artery thereby increasing the likelihood of tearing the artery. Conversely, a stent of sufficient flexibility to traverse the sharp bend in the right coronary artery is susceptible to recoil in the ostial right coronary artery. Accordingly, to the knowledge of the inventors, there is no known effective manner by which a stent may be used to treat a conventional ostial stenosis.
Accordingly, it would be desirable to have an improved stent which overcomes these disadvantages. It would also be desirable if such stent was relatively easy to implant. It would be further desirable if such a stent were capable of being uniformly expanded at relatively low pressure while obviating or mitigating longitudinal shrinkage thereof. It would be further desirable if such a stent were not susceptible to asymmetric internal coverage of the body passageway, a problem associated with “coil”-type stents—see, for example, U.S. Pat. No. 5,282,824 (Gianturco). It would be further desirable if such a stent was not susceptible to movement along the longitudinal axis of the body passageway during or after implantation. It would be further desirable if such a stent was characterized by a desirable balance of lateral flexibility in the unexpanded state and radial rigidity in the expanded state.